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StartOfGDT:
    zerodescriptor:
        .quad   0
    OScode:
        .quad   0x00cf9a000000ffff
    OSdata:
        .quad   0x00cf92000000ffff
    OSstack:
        .quad   0x00C0960000000001
    GDTend:

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StartOfGDT:
    ...
    OSstack:
        .quad   0x00C0960000010005 /*the limit probably can be anything, I did not managed to put the vm to guru meditation*/
    GDTend:

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.code16
/* add a jmp to set the cs to a known value. */
_start:
        ljmp $0x0, $begin
begin:
        cli;
        cld;

        xorw %ax, %ax;
        movw %ax, %ds;
        movw %ax, %es;

        lgdt bt_gdtr;

        movl %cr0, %eax;
        orw $1, %ax;
        movl %eax, %cr0;

        ljmp $0x8, $pmode;

.balign 8
bt_gdt:
        .quad 0
        .quad 0x00cf9a000000ffff
        .quad 0x00cf92000000ffff
        .quad 0x00c0960000000005
bt_gdtr:
        .word bt_gdtr - bt_gdt - 1
        .int bt_gdt

.code32
pmode:
        movw $0x10, %ax;
        movw %ax, %ds;
        movw %ax, %es;
        movw $0x18, %ax;
        movw %ax, %ss;
        xorl %esp, %esp;

        pushl $0xabcdabcd;
        popl %eax;

        /* EAX here is NOT 0xabcdabcd, but is the contents of the 
         * linear address 0xfffffffc.
         *
         * The pushl insn, must have attempted a write to linear 0xfffffffc,
         * but that address probably maps the BIOS read-only memory.
         */
1:
        jmp 1b;

        . = _start + 510
        .byte 0x55
        .byte 0xaa

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as a.s
ld -Ttext 0x7c00 --oformat=binary a.out -o a.bin
dd conv=notrunc if=a.bin of=floppy.img

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    mov     $STACK_BASE,%ebx
    movl    $STACK_LIMIT,%ecx
    movl    $4096, %eax
    mull    %ecx
    addl    %ebx,%eax
    sub     $16,%eax
    mov     %eax,%esp

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 mov 0x1000, %eax
 mov %eax, %esp

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cli
jmp     _start

memory_descriptor:
    .rep 12
    .byte 0
    .endr

oldGDTdescriptor:
    .word   0
    .int    0

GDTdescriptor:
    .word   0
    .int    0

.globl _start
_start:
    call    get_gdt_base
    lea     GDTdescriptor, %ebx
    movl    %eax, %ds:2(%ebx)
    lea     memory_descriptor, %eax
    sgdt    (oldGDTdescriptor)
    push    %eax
    call    create_base_gdt
    movw    $8, %dx
    mulw    %dx
    decw    %ax
    movw    %ax, %ds:(%ebx)
    lgdt    (GDTdescriptor)
    movl    $0x10,%eax
    mov     %ax,%ds
    mov     %ax,%es
    mov     %ax,%fs
    mov     %ax,%gs
    mov     %ax,%ss
    jmp     $8,$next
    next:
    movl    $0x3FFE0, %eax
    movl    %eax, %esp
    call    main

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0x0
/*code segment*/
0x00C09A0000008000
/*data segment*/
0x00C0920000008000